&lt;Industrial Application Field&gt;
The present invention relates to a sensor for measuring the degree of flatness of a wafer, disc or the like capable of measuring nearly edge peripheries thereof.
In regard to substrates of semiconductor wafer, optical disc, hard disc and others, conditions in the degree of flatness of the substrates are taken as important. In particular, in the wafer of semiconductor where many chips are cut from a one sheet of wafer, it is desirable to use the wafer nearly the edge thereof as possible in order to widen an available area of the wafer to be chips. For example, when chips of 25 mm.times.40 mm in size are cut from the wafer of 300 mm diameter, in case of leaving a remainder of 3 mm, number of 50 devices can be produced, but if a remainder to be left is 1 mm, 54 devices may be got out.
For using the wafer including the edge periphery to yield chip products, it is required to measure the degree of flatness of the wafer or disc up to the edges as nearly as possible. In the present invention, the degree of flatness is defined in term of the deviation in thickness from the surface to the reverse side of the wafer or disc.
When the degree of flatness of the semiconductor wafer is measured, sensors 2u, 2d are faced to both sides of the wafer 1 turnably held as shown in FIG. 1, and distances Du, Dd from the sensors 2u, 2d to the surface of the wafer 1 are measured. As the distance Dp between the sensors 2u, 2d is in advance known, the thickness D of the wafer 1 is given as D=Dp-(Du+Dd). If the sensors 2u, 2d are moved to the radius of the wafer 1 as rotating the wafer 1, the moving loci of the sensors 2u, 2d are made helical above the wafer 1, and the thickness D over an almost overall surface of the wafer 1 may be measured and the flatness is calculated.
Ones known as the sensors 2u, 2d are a sensor of capacitance for measuring variations in capacitance between the wafer 1 and the sensor 2u, 2d, or an optical sensor for measuring a distance from the wafer 1 to the sensors 2u, 2d by means of a laser beam irradiated to and reflected on the surface of the wafer 1.
&lt;Problems to be Solved by the Invention&gt;
Depending on the sensor of capacitance, a measured result is obtained as an average value in the surface area of the wafer which the measuring part of the sensor opposes and it has a high value in reproductivity. If a sensor whose measuring part has a large scope is used, a time of operating the whole surface of the wafer is saved and the reproductivity is further heightened.
However, the wafer to be measured is chamfered and inclined at the edge part thereof. If the measuring part of the sensor faces this inclined portion, or one portion of the measuring part deviates from the edge, a measured value calculated by the capacitance is less than actual data. It is, therefore, necessary to oppose the measuring part of the sensor to the surface of the wafer so that the measuring part does not enter the edge part. Namely, the degree of flatness of the wafer is measured, leaving an insensitive zone depending on the area of the measuring part and existing around the periphery of the wafer, and such a part excepting the insensitive zone is cut to chips.
For increasing the significant area of the wafer to be used as chips, the width of the insensitive zone present at the wafer periphery should be lessened as narrow as possible. Specifically, assuming that the wafer of the chamfered width being 0.5 mm is measured with the sensor of capacitance of a size being 4 mm.times.4 mm, the reliability of data is lowered in the peripheral part inside 2.5 mm or more than the edge. This system can be employed in the present exception of the edge being determined at 3 mm, but cannot be responded to an inclination that the exception of the edge is made smaller for increasing the yield.
On the other hand, the optical sensor is suitable for measuring a minute small area, and can measure nearly the edge. However since an object to be measured is a spot of about 1 to 2 .mu.m diameter, it is practically impossible to measure the full surface of the wafer, and several points in the wafer surface cannot but be sampling-measured. Therefore, the reliability or reproductivity of measured results are low.
Such problem occurs not only in the semiconductor wafer but also in the optical disc, hard disc or such where the surface condition is regarded as important.